Titanium metal and many of its alloys (e.g., Ti-6Al-2Zr-2Mo and Ti-6Al-4V) present a high strength-weight ratio which is maintained at high temperatures. Titanium metal and its alloys also have exceptional corrosion resistance. These characteristics are very desirable and have been the principal cause for the rapid growth of the titanium industry from the 1950's to the present. The aerospace industry has been the major consumer of titanium and titanium alloys for use in airframes and engine components. Non-aerospace applications include medical components, steam turbine blades, superconductors, missiles, submarine hulls, chemical apparatus, and other products where corrosion is a concern.
Titanium and titanium alloys possess physical properties which make these materials difficult to mill. Hence, cutting inserts used to mill these materials face special challenges which require the careful selection of the appropriate cutting insert.
In milling, there are repeated mechanical shocks due to the interrupted nature of the process. The mechanical shock may also produce cracks that result in microchipping of the cutting edge of the cutting insert.
The interrupted cutting process of the milling operation also causes thermal shocks since the cutting insert repeatedly heats up when in contact with the workpiece and cools down when removed from contact with the workpiece. In the milling of titanium and its alloys the magnitude of these thermal shocks is especially severe because of the typically high temperatures, and the accompanying large temperature differential between the high temperature and the low temperature of the cutting insert associated with a milling operation. Like with mechanical shock, thermal shock may produce cracks that result in microchipping of the cutting edge of the cutting insert.
Another reason that titanium and its alloys are difficult to mill is that they have a low thermal conductivity which worsens the ability to transfer heat into the workpiece and the chips and away from the cutting insert. This is especially true when one considers that when milling titanium and its alloys the chip travels across a surface of the cutting insert at a relatively fast speed, i.e., at a speed two to three times faster than when machining steel, with only a small area of contact between the chip and the surface of the cutting insert. Such a circumstance produces considerable heat-producing shearing of the chip which results in high temperatures (e.g., about 1093.degree. C. (2000.degree. F.) at the interface between the cutting insert and the chip).
At interface temperatures of about 500.degree. C. (932.degree. F.) and above, which includes 1093.degree. C. (2000.degree. F.), titanium and titanium alloys are typically chemically reactive with the cutting insert material, as well as with the nitrogen and the oxygen in the air. This chemical reactivity typically increases with an increase in temperature so that at high cutting insert-chip interface temperatures such as 1093.degree. C. the titanium workpiece is very reactive with the cutting insert and its surrounding environment.
Because of the high temperatures generated at the cutting insert-chip interface and the highly reactive nature of titanium and its alloys, diffusion of elements of the cutting insert into the chips (of the workpiece material) may cause cratering of the cutting insert.
The cutting insert-chip interface may be under pressures in the order of about 1.38 to 2.07 gigapascal (200,000 psi to 300,000 psi). These high stresses at the cutting edge of the cutting insert may lead to deformation and fracture of the cutting insert.
Thus, it is apparent that a cutting insert for use in milling titanium and its alloys should possess certain physical and mechanical properties which will enable it to address the challenges inherent in the milling of titanium and its alloys. More specifically, the cutting insert should be able to resist mechanical shocks and thermal shocks which are inherent in a milling operation. The cutting insert should be able to withstand the high temperature at the cutting insert-chip interface so as to minimize cratering. The cutting insert should also be able to withstand the high stresses which may lead to deformation, especially at the high operating temperatures associated with milling titanium and its alloys.